Inconsistent Line position between filles

Hello

I have attached a screen shot of two artboards. Each artboard has a horizontal line at 190mm from the top (position Y). When I place the two artboards next to each other, both at 33.3%, the position of the lines is visibly different. I would expect them both to look exactly the same. Can anyoone explain why the lines appear to be at a different position on each page, even though they were placed at exactly the same position?